Multi-zone sound field reproduction is a technique that aims at providing an individual sound environment to each listener without physically isolated regions or the use of headphones. With the increased need for personalized sound environments in the fast growing entertainment and communication field, spatial multi-zone sound field reproduction over an extended region of open space has conducted to the definition of several solutions, such as described by M. Poletti “An investigation of 2D multizone surround sound system” Proc. AES 125th Convention Audio Eng. Society, 2008; N. Radmanesh and I. S. Burnett “Reproduction of independent narrowband soundfields in a multizone surround system and its extension to speech signal sources” Proc. IEEE ICASSP, 11:598-610, 2011 and Y. J. Wu and T. D. Abhayapala “Spatial multizone soundfield reproduction” Proc. IEEE ICASSP, pages 93-96, 2009.
Spatial multi-zone sound field reproduction is a complex and challenging problem in the area of acoustic signal processing. The key objective is to provide the listener with a good sense of localization by precisely reproducing the desired sound field in the designated bright zone, while also controlling the acoustical brightness contrast between the bright zone and quiet zone. The region that features high acoustical brightness at a specified frequency is defined as the bright zone and the region that features low acoustical brightness is defined as the quiet zone. The acoustical brightness of a zone at a particular frequency is defined as the space-averaged potential energy density at that frequency. The acoustic energy density is proportional to the square of the pressure complex magnitude, which is the sound field magnitude squared. Ideally the acoustic energy density of a quiet zone is set to be zero, however, in practice it is generally small relative to other zones. In that case, the objective is to achieve an acoustical brightness contrast, which is defined by the power ratio between quiet and bright zones.
Using a linear loudspeaker array consisting of sixteen speakers, Ivan Tashev, Jasha Droppo and Mike Seltzer have demonstrated that sound waves cancel each other out in one area and become amplified in another. Someone stepping even a few paces to the side of the designated sound field can not hear the music. A preliminary theoretical study was performed in J. Daniel, R. Nicol, and S. Moreau “Further investigations of high order ambisonics and wavefield synthesis for holophonic sound imaging” Proc. AES 114th Convention Audio Eng. Society, 51:425, 2003, which introduced higher order ambisonics (HOA) to reproduce sound fields in multi-zones on the basis of mode matching. In 2008, Poletti proposed an alternative approach using least-squares matching to generate a 2-dimensional (2-D) monochromatic sound field in a multi-zone surround system. This was based on the computation of a circular loudspeaker aperture function which allows for a sound source positioned within or on a ring of speakers. Further investigation was made by N. Radmanesh and I. S. Burnett to extend the work to two multi-frequency sources and then to narrowband speech signals.
However, none of the activities mentioned above provides a precise control on the sound leaked from one zone into other specified zones. In T. Betlehem and P. Teal “A constrained optimization approach for multizone surround sound” Proc. IEEE ICASSP, pages 437-440, 2011, a method was proposed to control the sound in each zone independently, while also controlling the leakage into other listeners' zones. A constrained optimization similar to P. D. Teal, T. Betlehem, and M. Poletti “An algorithm for power constrained holographic reproduction of sound” Proc. IEEE ICASSP, pages 101-104, March 2010, for determining the loudspeaker weights that minimize the mean square error (MSE) of reproduction in the control region was used. They incorporated a constraint on the summed square value of the loudspeaker weights to improve the system robustness. A method was proposed in J. W. Choi and Y. H. Kim “Generation of an acoustically bright zone with an illuminated region using multiple sources” JASA, 111:1695-1700, 2002, to make an acoustically bright zone (the zone of high acoustic potential energy) by using multiple control sources at a particular frequency. An acoustic contrast control method was introduced to maximize the acoustical brightness contrast between two zones (bright and quiet zones). A sound focused personal audio system for a mono sound was implemented as an example application and a pressure difference of up to 20 decibels (dB) between the bright and dark zone was demonstrated. In J.-Y. Park, J.-H. Chang, Y-H. Kim, and Y. Park “Personal stereophonic system using loudspeakers: feasibility study” International Conference on Control, Automation and Systems, October 2008, the acoustic contrast control method was further applied to a personal stereophonic system and the results demonstrated that a channel separation of over 20 dB can be obtained in the bright zone chosen around each ear. These methods are limited to the control of the acoustic energy contrast between two different zones and the outcome of this approach fails to control the sound field. Indeed, they do not provide a sense of localization for the listener in the bright zone.
In Y. J. Wu and T. D. Abhayapala “Spatial multizone soundfield reproduction” Proc. IEEE ICASSP, pages 93-96, 2009, a framework was proposed to recreate multiple 2-D sound fields at different locations within a single circular loudspeaker array by cylindrical harmonics expansions. They derived the desired global sound field by translating individual desired sound fields to a single global co-ordinate system and applying appropriate angular window functions. An improved method of using spatial band stop filtering over the quiet zone to suppress the leakage from the nearby desired sound field was proposed in Y. Wu and T. Abhayapala “Multizone 2D soundfield reproduction via spatial band stop filters” IEEE WASPAA, pages 309-312, 2009. However, both of these two methods were based on the idea of canceling the undesirable effects on the other zones by using extra spatial modes (harmonics). The drawback for this approach is that it is only able to create quiet zones outside the designated reproduction region, which renders the method not useful for practical applications. The reproduction region defines the total control zone of interest for the rendering of a desired sound field. Only the bright zone can be included in this zone of interest, the quiet zone can only be obtained outside this reproduction region. This reproduction region is at least delimited by the loudspeakers and usually limited to a small area.
The methods described in prior art do not provide the listener with a good sense of localization by precisely reproducing the desired sound field in the designated bright zone, while also controlling the acoustical brightness contrast between the bright zone and quiet zone in an efficient way. Prior art can only partly achieve this goal by either reconstructing a sound field or providing acoustical brightness contrast between two zones without localization information. T. Betlehem, P. D. Teal “A constrained optimization approach for multi-zone surround sound” Proc. IEEE ICASSP, pages 437-440, 2011 has described a method to achieve both acoustical brightness contrast and sound field reconstruction based on convex optimization, but the computational complexity of such method makes it hardly implementable in practical applications.